1. Field of the Invention
The present invention relates to a switching power supply device that includes a switching element on a primary side and a rectifier circuit on a secondary side and that performs power transmission by using the electromagnetic field resonance phenomenon.
2. Description of the Related Art
With recent progress toward compact and lightweight electronic devices, market demand for high-efficiency, compact, and lightweight switching power supplies has increasingly grown. For example, current resonant half-bridge converters that cause a transformer to operate in response to a sinusoidal resonant current flowing therethrough by using an LC resonance phenomenon have been increasingly put into practical use in markets of goods such as flat-screen televisions with comparatively moderated output current ripple characteristics, while taking advantage of the feature of high efficiency.
For example, an LC series resonant DC-DC converter is disclosed in Japanese Unexamined Patent Application Publication No. 9-308243. FIG. 1 is a basic circuit diagram of a switching power supply device in Japanese Unexamined Patent Application Publication No. 9-308243. The disclosed switching power supply device is a current resonant type half-bridge DC/DC converter, and an LC resonant circuit composed of an inductor Lr and a capacitor Cr and two switching elements Q1 and Q2 are connected to a primary winding np of a transformer T. A rectifying and smoothing circuit composed of diodes Ds1 and Ds2 and a capacitor Co is constructed at secondary windings ns1 and ns2 of the transformer T.
With the configuration described above, the switching elements Q1 and Q2 are alternately turned on and off with a dead time, and the current waveform flowing through the transformer T is a sinusoidal resonant waveform. Further, power is transmitted from the primary side to the secondary side during both on periods and off periods of the two switching elements Q1 and Q2.
However, in the switching power supply device of Japanese Unexamined Patent Application Publication No. 9-308243, the transformer is used as an insulating transformer that utilizes electromagnetic induction, and is merely utilized as a transformer that utilizes magnetic coupling. In a transformer that utilizes electromagnetic induction, the magnetic flux generated by a current flowing through the primary winding is linked to the secondary winding to cause a current to flow through the secondary winding, and efficient conversion from electricity to magnetism and further to electricity is important. In general, the proportion of the magnetic fluxes linked to the secondary winding in the magnetic fluxes generated by the current flowing through the primary winding is called the degree of (magnetic) coupling. In a transformer that utilizes electromagnetic induction, it is important to increase the degree of magnetic coupling in order to increase power conversion efficiency. However, the degree of magnetic coupling of the transformer may be difficult to increase because magnetic saturation is prevented or because of physical constraints, resulting in power conversion efficiency being reduced.
In the switching power supply device of Japanese Unexamined Patent Application Publication No. 9-308243, furthermore, since frequency control and PFM (Pulse Frequency Modulation) control are used for control of its output, it is necessary to design a smoothing circuit in accordance with the minimum operating frequency, which hinders compactness. In addition, in view of operation on the order of MHz for miniaturization of magnetic components, changes in operating frequency result in a significant problem in terms of the controllability of output, EMC (electromagnetic compatibility), and so forth.